The subject matter disclosed herein relates to ultrasonic testing systems.
Several industries (e.g., oil and gas, refinery, chemical, power generation) require the transport of fluid through pipes. Nondestructive testing systems are placed on the outer surface of these pipes to monitor corrosion (or erosion) of the pipes, including corrosion on the interior of pipe walls. In some nondestructive testing systems, the probe or other nondestructive testing device is permanently coupled to the outer surface of the pipe to continuously monitor corrosion at that location to determine pipe corrosion rates and to determine whether that pipe location is in need of preventative maintenance to prevent a pipe failure. In other nondestructive testing systems, the probe is portable and can be moved along the outer surface of the pipe.
One example of a nondestructive testing system used to monitor corrosion of a pipe is an ultrasonic testing system. When conducting ultrasonic testing of a pipe, an ultrasonic signal is emitted from a probe coupled to the outer surface of the pipe and passed through the pipe. As the ultrasonic signal passes into and through the pipe, various reflections called echoes are reflected back to the probe as the ultrasonic signal interacts with the outer surface of the pipe, internal structures, voids or occlusions within the pipe, and with the inner surface (or back wall) of the pipe. The echo signals can be displayed on a screen with echo amplitudes appearing as vertical traces and time of flight or distance as horizontal traces. By tracking the time difference between the transmission of the ultrasonic signal and the receipt of the echoes, various characteristics of the pipe can be determined, including pipe thickness. Knowing the time of flight of the ultrasonic signal from the outer surface of the pipe to the inner surface of the pipe and then back to the outer surface of the pipe, as well as the speed of sound in the material that the pipe is made from (e.g., 5,800 m/s for stainless steel 316L) enables determination of the thickness of the pipe. If the thickness of the pipe at the location of the ultrasonic testing system decreases over time (e.g., as would be shown be a reduction in the time of flight of the back wall echo), this can be an indication of corrosion.
In order to make highly accurate thickness measurements, the time of flight measurements must also be highly accurate. The accuracy of time of flight measurements can be negatively impacted by performance variation caused by temperature variation of components of the ultrasonic testing unit. For example, the time (propagation) delay between the time that the logic circuit outputs a trigger to the pulser and the time that the ultrasonic signal is actually fired (pulse delay) varies based upon the temperature of the logic circuit (e.g., at lower temperatures, the time delay is shorter than the time delay at higher temperatures). Similarly, the frequency of the clock signal that determines, e.g., the timing of the sampling of the received ultrasonic signal or the time of flight measurement also varies based upon the temperature of the clock oscillator (e.g., at lower temperatures, the frequency is lower (the period is longer) than the frequency at higher temperatures). These temperature variations can result in different time of flight measurements depending on the temperature of the logic circuit and the clock oscillator that can result in inaccurate thickness measurements. For example, if the time of flight measured when the logic circuit is at +85° C. is 30 ns longer than the time of flight measured when the logic circuit is at −40° C., the thickness measurement at the higher temperature would be approximately 0.087 mm larger than at the lower temperature, even though the actual thickness had not changed.
In order to account for the differences in time of flight (and thickness) measurements resulting from performance variation caused by temperature variation in the logic circuits or clock oscillators, the ultrasonic testing system must be calibrated often. In those permanent installations where frequent calibration is not practical, heating or cooling devices can be installed in the ultrasonic testing system to maintain a consistent temperature for the components of the ultrasonic testing system, increasing the cost and energy demands of the inspection system.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.